In vehicles, an automatic transmission is used to transmit torque produced by an engine to drive wheels. Because the engine produces varying magnitudes of torque that must be matched to the load imposed on the vehicle through the drive wheels, the automatic transmission attempts to transmit engine torque efficiently without wasting fuel or damaging the engine and driveline. To transmit the drive torque, the automatic transmission controls the amount of torque delivered to the drive wheels by a combination of a torque converter, lock-up clutch, multi-speed geared transmission, and hydraulic control system, including one or more valves and fluid pumps, for the aforementioned components.
In controlling the automatic transmission, the hydraulic control system principally relies upon two parametric inputs to determine the appropriate magnitude of torque to be transmitted from the engine to the drive wheels: (1) the output speed of the vehicle, and (2) a load imposed on the vehicle.
The output speed of the automatic transmission can be measured at the output shaft of the automatic transmission via a governor valve. The governor valve regulates a fluid pressure provided from the fluid pump to the governor valve as a function of the vehicle speed so that the faster the vehicle moves, the greater is the fluid pressure provided by the governor valve to other hydraulic valves of the hydraulic control system.
The load on the vehicle can be measured by sensing the vacuum developed on the intake of the engine or the magnitude in which a throttle of the engine is opened or closed. The sensed load is used to control a transmission throttle valve in the hydraulic control system. The transmission throttle valve regulates the fluid pressure provided from the fluid pump to the transmission throttle valve as a function of the load on the engine. That is, the greater the load is sensed (e.g., large throttle opening or low intake tract vacuum) the greater the fluid pressure is provided by the transmission throttle valve, and the lower the load (e.g., small throttle opening or high intake tract vacuum), the lower the fluid pressure provided by the transmission throttle valve.
To control the transmission throttle valve, it has been known to physically connect the transmission throttle valve to the engine throttle or to physically connect the transmission throttle valve to a vacuum modulator (e.g., a vacuum actuated mechanism) that moves the transmission throttle valve in relation to the engine vacuum generated in the intake tract or the opening of the engine throttle valve.
The fluid pressure provided by the governor valve and the fluid pressure provided by the transmission throttle valve are generally applied in opposition to each other in one or more hydraulic valves to regulate, among other functions, a line pressure in the hydraulic control system, shift timing and shift feel of the automatic transmission. That is, by sensing these two parametric inputs, the hydraulic control system regulates torque by selecting the appropriate torque ratio change (i.e., gear ratio) and the duration of the gear ratio change as the load and speed of the vehicle are changing.
To explain the operation of the transmission throttle valve, shift valves and governor valve in the simplified example of the known automatic transmission, it is assumed, for example, that the vehicle is traveling at a constant speed in a first gear of the transmission such that the transmission throttle fluid pressure and the governor fluid pressure are provided in opposition to each other in a shift valve. When the throttle and governor pressures are at respective preset levels in the shift valve, the first gear is maintained; when the governor fluid pressure is greater than a preset amount to the transmission throttle pressure, the shift valve allows a change from the first gear to the second gear; and when the transmission throttle pressure is greater than the governor pressure by a preset amount, the shift valve prevents a change in gear ratio to provide increased acceleration or speed. In this example, when throttle is opened quickly to a full open position, the transmission throttle valve is moved (due to its physical connection to the engine throttle valve) to provide increased hydraulic fluid pressure to the shift valve in relation to the governor pressure. As such, a shift from the first gear to the second gear is delayed. On the other hand, if the speed of the vehicle has increased while the load (e.g., throttle opening) remains constant, the fluid pressure from the governor valve will be increased, relative to the throttle pressure, as delivered to the shift valve, thereby causing a shift in the gear ratio.
As discussed above, the known transmission throttle valve is actuated by a physical connection to the engine throttle valve. That is, the known transmission throttle valve is moved by a physical connection, e.g., a cable connection, with the accelerator pedal via the engine throttle. The transmission throttle valve cable connection allows the driver to directly influence the operation of the transmission throttle valve by a fixed physical relationship via the cable. Thus, the known cable connection for the transmission throttle valve is suited for its purpose in permitting the driver to influence the operation of the automatic transmission.
However, the known cable connection is believed to be disadvantageous in many aspects. For example, as the known cable connection uses a cable, e.g., a Bowden type, the cable tends to stretch over time, resulting in miscalibration of the engine throttle valve to transmission throttle valve relationship, causing delayed shift timing or improper shift timing, duration or shift feel. Further, once the cable has stretched, attempts to recalibrate the transmission throttle valve to the engine throttle valve by adjusting the cable may not be successful, leading to delayed downshifts, short shifting, delayed or hard upshifts, all of which may lead to premature clutch failure in the automatic transmission.
Furthermore, vehicles increasingly are manufactured with electronic drive-by-wire throttle valves. These electronic engine throttle valves may not operate in a manner consistent with the intended operation of the transmission throttle valve for the known automatic transmission, and therefore may lead to shifting behaviors that are not desirable. Moreover, in order to change shift duration, timing and feel, it is believed that modifications of the transmission throttle valve, valve body or the throttle cable actuator may be necessary. Additionally, it is believed that the known transmission is insufficiently precise in maintaining a constant cruise speed when placed in a cruise control mode. Consequently, it would be advantageous to provide for a transmission throttle valve mechanism that would overcome these perceived deficiencies in the known cable actuator.